1. Field of the Invention
The present invention relates to switching regulators of self-oscillating mode (hereinafter occasionally referred to as an “RCC (Ringing Choke Converter)” mode), which include a control winding that allows a voltage to be induced therein by a magnetic flux created in the primary winding of a high frequency transformer, a capacitor charged by the voltage induced in the control winding, a transistor that is turned off when the voltage across the capacitor reaches a specified level, and a switching element driven by the transistor to switch on or off the input current through the primary winding of the high frequency transformer.
2. Description of the Related Art
In many of the appliances with image forming devices, such as copiers, the DC loads are powered by: decreasing by a transformer an AC voltage input from a commercial AC power supply; converting the voltage into a DC voltage by a rectifier circuit and a smoothing circuit; and then passing the DC voltage through a low-voltage power supply circuit serving as a regulator, thereby stabilizing the DC voltage at a specified level.
In recent years, switching mode low-voltage power supply circuits utilizing switching elements have been in the mainstream. In particular, switching regulators of self-oscillating mode are widely used because of their simplicity in circuit structure and cheapness.
Examples of this kind of switching regulator are disclosed in Japanese Unexamined Patent Publication Nos. 5-304778 and 2004-320856.
FIG. 5 shows an RCC mode switching regulator having a control winding N3 that allows a voltage to be induced therein by a magnetic flux created in a primary winding N1 of a high frequency transformer T, a capacitor C3 charged by the voltage induced in the control winding N3, a transistor Q2 that is turned off when the voltage across the capacitor C3 reaches a specified level, and a switching element Q1 driven by the transistor Q2 to switch on or off the input current through the primary winding N1.
The RCC mode switching regulator is influenced by ambient temperature and the operating temperature of the circuit, causing fluctuations in the DC amplification factor hfe of the transistor Q2 and the capacitance of the capacitor C3. This posed problematic fluctuations in the oscillating frequency, that is, the switching frequency of the switching element Q1, even at a constant load current.
For example, if ambient temperature rises, the capacitance of the capacitor C3 decreases and the DC amplification factor hfe of the transistor Q2 increases at the same time, which makes faster the switching operation of the input current through the primary winding N1 by the switching element Q1, which is driven by the transistor Q2.
The switching regulator also experiences what is called noise terminal voltage generated at a frequency of an integer multiple of the oscillating frequency. This voltage refers to external transmission of a noise occurring in the circuit through the AC line.
The noise terminal voltage is imposed limits by a safety standard, among which those for 150 kHz through approximately 500 kHz are set to decrease logarithmically.
At an approximately constant load current such as when the appliance is on stand-by, the switching frequency is usually stable at a specified frequency and thus the noise terminal voltage is below the above-mentioned limits, but, as described above, the switching frequency might drift because of the influence of ambient temperature, resulting a possibility of deviation from the limits.
In addition, if the switching frequency increases, so does the switching loss resulting in the problem of deteriorated conversion efficiency of the switching regulator.
These circumstances necessitate providing the switching regulator with a cooling fan and selecting circuit elements with superior temperature properties such as for the capacitor, in order to keep ambient temperature below specified temperatures. However, this causes an increase in cost associated with the enlarged occupied area as well as increased costs for the elements. There are also problems including noises from the cooling fan.